|Effekte erhöhter UV-Strahlung auf die Photosynthese arktisch/kalt-gemäßigter Makroalgen = Effects of enhanced UV-radiation on photosynthesis of Arctic/cold-temperate macroalgae|
Bischof, K. (2000). Effekte erhöhter UV-Strahlung auf die Photosynthese arktisch/kalt-gemäßigter Makroalgen = Effects of enhanced UV-radiation on photosynthesis of Arctic/cold-temperate macroalgae. Ber. Polarforsch. Meeresforsch. 375: 1-88
In: Berichte zur Polar- und Meeresforschung = Reports on Polar and Marine Research. Alfred-Wegener-Institut für Polar- und Meeresforschung: Bremerhaven. ISSN 1618-3193, more
Arctic macroalgae are subjected to strong seasonal and daily changes in the radiation climate. They are exposed to six months of darkness during the polar night, but also suddenly exposed to high radiation in spring after the break-up of the sea ice, especially during low tide at high water transparency. Elevated levels of UVB radiation (UVB), resulting from stratospheric ozone depletion, also contribute to high radiation stress. The investigations presented here were conducted to study the effects of enhanced UV-radiation (UVR) on the physiology of Arctic/cold temperate macroalgae in the laboratory and in the field. The results present a basis for predicting future changes within Arctic coastal ecosystems with respect to increasing UVB levels. Radiation conditions in the Arctic change dramatically with the seasons. At the study site (Kongsfjord, Spitsbergen) at approx. 80°North, the polar night lasts from mid October until mid February, the polar day from mid April to mid August. Maximal irradiances on the surface are about 1300 µmol photons m-²s-¹ of photosynthetically active radiation (PAR; 400-700 nm), 19 Wm-² UVA (320-400 nm) and 1,09 Wm-² UVB (280-320 nm). The UVB irradiance is strongly dependent on the actual ozone concentration in the stratosphere, as confirmed spectrometrically by radiation measurements. The light climate in the water column is highly variable. Dissolved organic matter (DOM), sediment, phytoplankton blooms, as well as the tidal cycle determine additionally the in situ radiation conditions of macroalgae. The deepest penetration of UVB into the water column of the Kongsfjord has been determined at 10m depth. Numerous biological processes, such as photosynthesis, are impaired by UVB. The degree of UVR induced inhibition of photosynthesis as well as the potential to acclimate to changing radiation conditions is species dependent, as demonstrated by field experiments on different algal species which were collected in deeper waters and transplanted to shallow waters. Maximal quantum yield of photosynthesis acclimates rapidly to increased radiation conditions in species characteristic for the upper sublittoral zone (e.g. Palmaria palmata), while photosynthesis in species from deeper waters (e.g. Phycodrys rubens, Ptilota plumosa) is significantly impaired. These experiments indicate that the ability to acclimate to irradiance changes is genetically fixed. Different processes involved in photosynthesis are impaired by UVR exposure. The adverse effects of UVR on the Calvin cycle enzymes ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) and glyceraldehyde-3-phosphate dehydrogenase (G3PDH) in marine macroalgae were studied for the first time. RubisCO is particularly sensitive to UVR exposure. Reduced photosynthetic electron transport rates may be related to decreasing RubisCO activity, which is, partly a result of the degradation of the protein. In the brown alga Alaria esculenta, the formation of a high molecular weight polypetide was observed during UVR exposure, in parallel with decreasing concentration of the large and small subunit of RubisCO, indicating an aggregation of degraded protein. In contrast to RubisCO, G3PDH is more resistant to UVR exposure. The different sensitivity of photosynthetic reactions reflects the zonation patterns of the species examined in the field. Exposure to UVR can cause detrimental effects on photosynthesis, and macroalgae have developed acclimation strategies to cope with the drastic changes in irradiance. Acclimation of photosynthesis to changing radiation was studied in the brown algae Laminaria saccharina, Alaria esculenta, Saccorhiza dermatodea, collected at different water depths. Maximal quantum yield of photosynthesis in specimens collected in greater water depth is significantly more strongly impaired than in shallow water algae of the same species and exposed to the same fluence of artificial UVR. The ability to acclimate to various radiation conditions seems to be necessary for growth over a wide range of water depths. The time course of acclimation of photosynthesis to enhanced levels of white light and UVR has been studied in the brown alga Alaria esculenta. Low light acclimated algae were collected in spring under sea ice and exposed to repeated exposure cycles of different radiation conditions. Maximal quantum yield acclimates significantly within a few days. During the first exposure cycle, photosynthesis is primarily impaired and recovery from inhibition proceeds slowly. However, after some days of treatment, the capacity for recovery is increasing significantly and inhibition is reduced. In samples previously acclimated to high levels of PAR and exposed to high PAR supplemented with UVR, no additional UVR-inhibition of photosynthesis occurs, but recovery proceeds significantly more slowly. This indicates that photoinhibition is predominantly caused by white light, whereas UVR slows down the recovery process. in situ experiments with the brown alga Laminaria saccharina and the red alga Palmaria palmata show, that photosynthesis is hardly impaired by UVB irradiation at the natural growth site of these species, as they are protected by the water column above. After irradiance is increased by transplanting the algae from 3 to 1 m water depth, the maximal quantum yield of photosynthesis acclimates stepwise. An additional UVR treatment in the laboratory does not further impair the algae, previously kept at 1 m water depth. In contrast, samples from greater depth are very sensitive to the artificial UVR treatment. A possible protective mechanism against increasing UVR is the synthesis of UVR screening compounds. In the Arctic endemic red alga Devaleraea ramentacea, the synthesis of UVR screening mycosporine-like amino acids (MAAs) has been studied. These substances are commonly found in various red algal species from shallow waters and are shown to provide partial protection of photosynthesis against UVR induced inhibition. In D. ramentacea the synthesis of MAAs is predominantly induced by the UVB component of the solar spectrum, as shown by UVR exclusion experiments in the field. The internal MAA concentration is determined by several factors all depending on solar radiation. Specimens from the same collecting site exhibit much higher MAA concentration when sampled in August (at the end of the Arctic summer) as when being sampled in May. The content of MAAs is also related to collection depth, with algae from shallow waters containing significantly higher concentrations of MAAs than deep water samples. There is also a marked gradient of MAA concentration within the thallus. The sun-exposed tips contain higher concentrations of MAAs than the shaded base. The respective concentration and composition of MAAs is species dependent. The accumulation of high concentrations of MAAs might be linked to the respective vertical distribution of species on the shore. This aspect was studied in the closely related red algal species Chondrus crispus and Mastocarpus stellatus from the island of Helgoland. Thalli of both species were collected from the same location and exposed to artificial UVR radiation. Photosynthesis in C. crispus responds more sensitively to UVR than M. stellatus, which might be related to the highly different MAA composition. M. stellatus contains up to 6-fold higher concentrations of MAAs than C. crispus, probably allowing M. stellatus to grow at locations more exposed to the sun. Different life history stages exhibit strong differences in UVR tolerance. In particular, brown algal zoospores are very sensitive to UVR exposure. Photosynthesis of spores is more strongly impaired by UVR than that of large sporophytes, and results in an increased mortality of spores. Spores of different species from the Arctic and Southern Spain and exposed to the same dose of UVR, show that the viability is species dependent. Spore mortality of species that are commonly growing in the lower sublittoral zone (e.g. Laminaria saccharina) is higher than from species growing in shallow waters (e..g. Chordaria flagelliformis). The mortality of Laminaria digitata spores is positively correlated with the formation of thymine dimers indicative for DNA damage. UVR irradiances in Southern Spain, commonly measured in water depths shallower than 7 m, induce mortality in spores of four species of the Laminariales. This indicates that the particularly high UVR sensitivity of zoospores might be a factor determining the vertical distribution of species in the field. The conclusion of this study is that UVR clearly has the potential to harm Arctic/cold temperate macroalgae. However, several acclimation and protective mechanisms are present in different species to counteract the negative effects. In species from the intertidal or upper sublittoral zone, efficient acclimation mechanisms have evolved to cope with the drastic changes in the ambient light climate. In contrast, algae from the deep sublittoral zone, and therefore generally not exposed to strong UVB, possess only limited capacities for acclimation. However, the knowledge of the species-dependent acclimation potential is not sufficient to predict how the plants will be affected by increasing UVB due to further ozone depletion in future.